water heater is leaking out of drain pipe into yard

The hot water heater in the garage has been running for a few hours. I went outside and against the house where the heater is, there is a PVC drain where hot water is leaking out in a steady stream. There is a hissing noise near where the hot water exits the hot water heater into the PVC pipe where I guess it is a pressure release valve? Not sure if this drain is from the drain pan? I'm far from knowledgeable in this area. Anything I should do? Is this something I can fix or do I need a pro?

One other note, we put in a new AC about 8 months back and they did work on the hot water heater when removing our old system. The hot water heater is about 14 years old. Should I call the AC guys first or is this probably unrelated?

Ok, just educated myself (ain't the internet grand ) . I opened the pressure release valve and let some water flow and the hissing noise as well as the draining water ceased. I'm hoping I just resolved some debris in the line, but should I change the pressure release valve to be safe or just monitor it to see if the problem comes back?

If you did not open that valve, it may have opened by itself which indicates a problem. That T&P valve can open itself because of either too hot (somewhere around 205-degrees) or too high pressure (over 150psi). If you have a PRV and an expansion tank, the expansion tank may need replacing. If the water wasn't excessively hot, I'd look for high pressure. You can check that with a cheap (about $10) pressure gauge you can attach to a hose bib, washing machine, or the drain valve of the WH (careful, it will get hot); or, with adapters, anywhere else in the water system you wish. Get one with a second, tattle tale hand to show peak pressure. If it is expansion, it will do it again after someone uses a bunch of hot water and the WH gets turned on. While the WH is on and no water is being used, the heated, expanded water has to go somewhere - and, that can be out the T&P valve in a closed system with a failed expansion tank.

Thanks for the detailed information. So far I'm monitoring it. We've taken a few showers and run the washing machine and I haven't seen it leak since last night. I haven't taken any pressure reading yet though.

If it is pressure, it will be most evident when you've used a lot of hot water, then, do not use ANY water while the WH is recovering. Otherwise, each time you use a little water, it will drop the pressure back down, and it may not get high enough to open the T&P valve on the WH. Depending on the WH, the thermostat setting, and your incoming cold water, the expansion is in the range of up to maybe a quart or so, often less. But, that's enough to be annoying plus, the T&P valve is not designed for regular use...it is a safety device that should not be regularly opened (as in daily - maybe check it once a year).